Currently LastPass supports the Yubikey in the limited capacity as a OTP second factor for authentication. Additionally, as I understand it, the static portion of the key is also used to encrypt the password file locally for offline access - curious to know if this is uniform across all versions of the LastPass app?

With iOS 13 Apple is providing support for 3rd party developers to not only read from NFC but also write - which means in theory the ability to use the Challenge-Response functionality of the Yubikey. For those unaware of Challenge-Response in this context basically you take your master password, "write it" to the yubikey which will hash it with a hardware secret on the Yubikey, and then is "read" out to the host device. This output is used to encrypt the password file. This is the way PasswordSafe and other password management apps add a solid layer of security.

With the inclusion of support by iOS 13 (iPhone 7+) LastPass won't have any hardware limitations to implement full Challenge-Response with the Yubikey.

Is this currently being developed along with the publicly available iOS 13 Beta?

In part this is somewhat determining if I will subscribe to LastPass premium or switch to a different password manager that is on the fore-front of end-user security.